Production of an industrial product through oxidation of an organic compound employs a method involving mixing an oxygen-containing substance referred to as an oxidant and an organic compound and conducting an oxidation reaction by using a catalyst, heat, or the like.
For example, industrial production of adipic acid as a raw material for nylon employs a method (nitric acid oxidation) of oxidizing cyclohexanone or cyclohexanol as a raw material by using a large amount of nitric acid. This method employs nitric acid, which is a strong acid, as an oxidant, and thus has problems such as an increased cost for a measure against corrosion of an apparatus and an increased cost for reduction of risks in production.
Industrial production of terephthalic acid as a raw material for a polyester employs a method (air oxidation) of oxidizing paraxylene as a raw material by using oxygen in air. This method employs cobalt, manganese, and a boron compound as catalysts, and thus has problems such as an increased cost for a measure against corrosion due to boron and an increased cost for reduction of an environmental load due to boron.
An example of a method of oxidizing a substance such as a silicon wafer is a method of bringing a substrate into contact with water containing an oxidizing substance dissolved therein. Examples of water containing an oxidizing substance dissolved therein include a hydrogen peroxide solution and ozone water.
The hydrogen peroxide solution is used for bleaching of paper or pulp, washing of a semiconductor, sterilization, disinfection, and the like by utilizing the oxidizing power thereof. The ozone water is used for sterilization of water supply and sewage system and sterilization of food and tableware, and is recently used for washing of a semiconductor and the like.
The hydrogen peroxide solution is relatively stable, and hydrogen peroxide remains for a relatively long period of time in waste water obtained after the hydrogen peroxide solution is used for various applications. Thus, the waste water of the hydrogen peroxide solution requires decomposition treatment for reducing an environmental load, and industrial use of a large amount of the hydrogen peroxide solution or the like causes a problem of an increased cost required for the decomposition treatment.
Meanwhile, the ozone water is relatively unstable and therefore decomposes spontaneously in a relatively short period of time. However, the ozone water is toxic to human body even in a low concentration and provides a heavy load on a material used for a piping system or the like, so that the ozone water requires decomposition treatment. Thus, the ozone water has a problem of causing a treatment cost. The ozone water is relatively unstable, and storage of the ozone water while maintaining the oxidizing power thereof for a long period of time is substantially impossible. That is, the ozone water is inconvenient in that a method of using a required amount of ozone water at a required time from a large amount of stored ozone water is impossible. Further, stable supply of ozone water after start of an ozone water production apparatus in a stopped state requires a certain period of time for start-up of the apparatus. In the case where intermittent use of ozone water is supposedly desired, an operating state of the ozone water production apparatus must be maintained at all times even during time requiring no ozone water if an apparatus start-up time is longer than time requiring no ozone water. Thus, a useless raw material cost or a useless running cost is inevitably caused.
Another example of the method of oxidizing a substance is a method employing a photocatalyst. The photocatalyst is a substance which exhibits a catalytic action under irradiation of light, and a typical example of the photocatalyst is titanium dioxide having an anatase crystal structure. Titanium dioxide exhibits an oxidizing action under irradiation of light having a wavelength shorter than about 380 nm. In the presence of a target substance to be oxidized in a gas phase, titanium dioxide is allowed to be present in the gas phase, and the gas phase is irradiated with light for oxidation of the target substance. In the presence of a target substance to be oxidized in water, titanium dioxide is allowed to be present in water, and water is irradiated with light for oxidation of the target substance.
Titanium dioxide is used for deodorization and decomposition of a malodorous component such as ammonia or formaldehyde, sterilization and purification of drink water or waste water, and the like by utilizing its oxidizing power as a photocatalyst. Titanium dioxide is often used in a thin film form fixed on an appropriate substrate surface for easy handling and basically exhibits an oxidizing action on a substance adsorbed on a surface of titanium dioxide alone. Thus, diffusion and adsorption of a substance to be oxidized itself from and to the surface of titanium dioxide in a gas phase or water control rate of oxidation, and use of titanium dioxide has a problem of a relatively slow oxidation rate.
In use of titanium dioxide in water, a water molecule adsorbed on the surface of titanium dioxide is oxidized to generate a hydroxy radial having oxidizing power. The hydroxy radical has a very short life and exists in adjacent to titanium dioxide very closely. Thus, no substantial oxidizing action of the hydroxy radical derived from diffusion of the hydroxy radical can be confirmed in water distant from titanium dioxide. In this way, the hydroxy radical differs from the hydrogen peroxide solution or ozone water and has a problem in that even when a solid substance exhibiting no solubility in water such as a semiconductor substrate is immersed in water containing titanium dioxide, a surface of the solid substance cannot be oxidized in the presence or absence photoirradiation. In the case where an attempt of increasing an oxidation rate is made by dispersing titanium dioxide in a form of fine powder in water, this case has a problem in that separation and recovery of titanium dioxide after oxidation treatment involves difficulties.
There is proposed an oxidation method employing nitrous oxide. Nitrous oxide (N2O) is a stable gas under normal temperature and normal pressure and is not decomposed by visible light. For example, Non-patent Document 1 describes properties of nitrous oxide. Nitrous oxide is known to decompose into a nitrogen molecule (N2) and atomic oxygen (O) under irradiation of light having a wavelength shorter than 240 nm. Non-patent Document 2 describes this phenomenon, for example.
Many researches have been conducted regarding oxidation of a target substance by using atomic oxygen (O) generated in a gas phase. For example, Non-patent Document 3 describes oxidation of a surface of an Si wafer.
Patent Document 1 discloses an invention involving: removing a natural oxide film on a surface of a silicon substrate with a diluted hydrofluoric acid solution; heating the silicon substrate to about 300° C.; brining an ultra pure oxygen gas into contact with the silicon substrate to form a silicon oxide film of about an intermolecular distance, and heating the silicon substrate to 900° C. to form an oxide film having a predetermined thickness. Oxidation of the silicon substrate can be conducted by using a solution containing oxygen and/or a molecule containing oxygen. The solution to be used may be a solution containing oxygen dissolved therein, a solution containing ozone dissolved therein, a hydrogen peroxide solution, a sulfuric acid/hydrogen peroxide aqueous solution, a hydrochloric acid/hydrogen peroxide aqueous solution, or an ammonia/hydrogen peroxide aqueous solution (see paragraph 0013 of Patent Document 1).
Patent Document 2 discloses a method of using nitrous oxide and forming a natural oxide on a surface of a semiconductor substrate with an oxygen atom generated from nitrous oxide. Formation of the oxide is conducted in a gas phase under pressure less than an atmospheric pressure.
Production of electronic components employs a technique of oxidation (hereinafter, referred to as local oxidation) and insulating a selected region on a conductive material for various purposes. Further, there is realized a technique of working a substrate by forming an oxide film layer having high chemical resistance through local oxidation, and using the oxide film layer as a mask for dissolving and removing a non-oxidized region with a chemical. Production of an ornament widely employs a technique of forming an oxidized region having a different color from that of a non-oxidized region or an oxidized region having improved coloring property than that of a non-oxidized region on a substrate surface as a pattern for obtaining an ornament with a very aesthetic appearance.
The local oxidized region is conventionally formed by a method involving: forming a resist pattern or a pattern of an oxidation resistant material in a photolithography step; using the pattern as a mask; and oxidizing a region not covered with the mask. Alternatively, the local oxidized region is conventionally formed by a method involving: forming an oxide layer on a substrate surface in advance; forming a resist pattern in a photolithography step; using the pattern as an etching mask; and dissolving and removing the oxide layer in a region not covered with the mask to obtain the oxide layer in a region covered and protected by the resist alone. FIG. 12 shows the local oxidation method employing the photolithography step conventionally used and employing the resist described above.
First, as shown in FIG. 12(a), a photoresist layer 71 is formed on a substrate 70 to be subjected to local oxidation by a spin coating method or roller coating method of a liquid resist, a laminating method of a dry film resist, or the like conventionally and often used.
Next, as shown in FIG. 12(b), the photoresist layer 71 is exposed to light through a mask (reticle) 72 having a predetermined pattern and then developed, to thereby form the photoresist layer 71 into a mask pattern having a predetermined shape as shown in FIG. 12(c).
Then, as shown in FIG. 12(d), a region on the substrate 70 corresponding to an opened part of the photoresist layer 71 is selectively oxidized, by using the photoresist layer 71 having a predetermined shape as a mask, by a wet chemical oxidation method employing a solution containing an oxidant such as a hydrogen peroxide solution or ozone water, an anodic oxidation method in an electrolytic solution, a dry oxidation method such as oxygen ion implantation or heat oxidation employing a quartz reaction tube, or the like, to thereby form an oxidized region 70a. 
After local oxidation, the resist layer 71 is removed through dry ash treatment employing plasma, wet treatment employing a resist removing liquid, or the like, to thereby obtain the substrate 70 having the desired oxidized region 70a formed thereon as shown in FIG. 12(e).
The oxidized region 70a may be formed to float (island) on the surface of the substrate 70 as shown in FIG. 12(e), or may be formed to reach a primary layer 73 as shown in FIG. 12(f) depending on the intended use.
For example, production of a semiconductor apparatus generally employs a technique of local oxidation of silicon (LOCOS) involving: forming a mask pattern of silicon nitride which is an oxidation resistant material on a silicon wafer in a photolithography step; and locally oxidizing and insulating silicon in a region not covered with silicon nitride by a dry oxidation method.
Patent Document 3 discloses a technique of forming a resist pattern on a metal thin film of aluminum, tantalum, or the like in a photolithography step and locally oxidizing and insulating the metal thin film in a region not covered with the resist pattern by an anodic oxidation method for dielectric isolation of the metal thin film in a matrix form in production of an active matrix-type reflective liquid crystal display apparatus.
Patent Document 4 discloses a technique of forming a resist pattern on a conductive material (such as iron, zinc, aluminum, aluminum alloy, titanium, tantalum, copper, copper alloy, silicon, silicon alloy, silver, zirconium, tungsten, chromium, or molybdenum) in a photolithography step, and locally oxidizing and insulating the conductive material in a region not covered with the resist pattern by an anodic oxidation method, a heat oxidation method, or an ion injection method for insulation treatment of a part of a leading electrode layer for leading a signal from a thin film magneto-resistance element excluding a leading electrode shape to form a pair of leading electrode parts as insulated parts in production of a thin film magneto-resistance effective head to be used for a magnetic information recording apparatus such as a hard disk.
Patent Document 5 discloses a technique of providing an oxide film on a surface of a silicon wafer in advance, forming a resist pattern in a photolithography step, removing the oxide film in a region not covered with the resist pattern with a hydrofluoric acid buffer solution, removing the resist in a mixed system of sulfuric acid and a hydrogen peroxide solution to form a pattern of a silicon oxide film, and etching a region not covered with the silicon oxide film with an alkali solution by using the silicon oxide film pattern as a mask in micromachining for producing a precision machine component, a microsensor, or the like employing a silicon wafer such as a semiconductor pressure sensor or an inkjet printer head.
Patent Document 6 discloses a technique of providing an oxide film on a surface of a substrate (such as aluminum, copper, manganese, silicon, magnesium, or zinc) in advance, forming a resist pattern in a photolithography step, removing the oxide film in a region not covered with the resist pattern with an alkali solution, and removing the resist to form an oxide film pattern in a defined region in a method of producing an ornament.
Patent Document 1: JP-B 3210370
Patent Document 2: JP-B 4-36456
Patent Document 3: JP-A 10-268359
Patent Document 4: JP-A 6-338035
Patent Document 5: JP-B 3525612
Patent Document 6: JP-A 2005-15898
Non-patent Document 1: Naotoshi Yamanouchi, Mitsuo Takeda, “Nitrous oxide”, High Pressure Gas, Vol. 13 No. 3 (1976) p 105-111
Non-patent Document 2: “Photochemical Reaction in Gas Phase”, edited by Chemical Society of Japan, Kagaku Sousetsu Muki-Hikari Kagaku Gakkai Shuppan Center, No. 39, (1983) p 14-38
Non-patent Document 3: K. Uno, A. Namiki, S. Zaima, T. Nakamura, N. Ohtake, “XPS Study of the Oxidation Process of Si(111) via Photochemical Decomposition of N2O by an DV Excimer Laser”, Surface Science, 193(1988), p 321-335